Bi-level printing technologies reproduce images by placing a series of marks or spots along selected points of a printing substrate. Such binary techniques may arrange spots to create dithered, gray scales. Manufacturers typically produce such gray scales using halftone screens. A halftone screen refers to a pattern of dots configured to create an image of varying tones and/or colors. The dots are spaced sufficiently-close such that an unaided human eye cannot distinguish between them. As such, the pattern will convey an overall impression of the desired image.
Dots are conventionally formed according to a threshold algorithm or spot function. More particularly, an algorithm executed by a raster image processor (RIP) may process “x” and “y” pixel coordinates to compare a local image value to a calculated threshold gray value. The results of the evaluation determine whether an image setter will assign a black spot to an addressable point that corresponds to the coordinates. In this manner, the algorithm may group points to form a dot pattern that makes up a screen.
Most screen making systems generate dots using PostScript processes. PostScript is an accepted industry standard description language capable of integrating text, line art and image data into a single document. The RIP may execute a PostScript spot function to generate an array of dot structures comprising threshold gray tones. The array may contain a continuous range of gray values from black to white. Conventional halftone algorithms may produce dot shapes within logically-constructed halftone cells.
A half-tone cell may comprise a square array of addressable, discrete points. The points are addressable via “x” and “y” coordinates of the cell. The coordinates may be scaled so that the cell extends from −1 to +1 in both the “x” and “y” directions. Of note, an operator may input dot pitch and angle requirements into the RIP. As such, the RIP may manipulate the orientation and spacing of halftone cells, while ensuring that each cell seamlessly tiles with neighboring cells at all four sides.
A dot pattern comprising the image may be output to film for conversion into a printing plate. Flexography is one printing process that utilizes such plates made from halftone techniques. Flexography is typically used for printing on paper, corrugated paperboard, and plastic materials. Flexography may utilize a photopolymer plate having projections and other contours that correspond to a halftone screen pattern. The plate may transfer ink onto a substrate using a simple stamping application. Specific examples of items printed with flexography may include: newspapers, milk cartons, frozen food and bread bags, as well as bottle labels.
Despite its wide application and versatility, plate printing processes, which include flexography, may remain prone to splotching and other undesirable ink distributions. In many instances, unsuccessful ink transfers are attributable to the texture of the print plate. Designers often struggle within the confines of restrictive PostScript code to create halftone screen configurations optimized for ink transfer. Other programming rules associated with RIP code may further constrain screen and halftone cell design options.
For instance, PostScript requires that each halftone cell align, or tile, on all sides with neighboring cells. Other programming constraints may limit the number of addressable points contained by a cell, and may prevent switching an activated pixel point from “on” to “off.” Such rules may limit the ability of screen designers to create optimized dot shapes that may be automatically generated by a RIP. Designer attempts to manipulate code sometimes fail to produce a threshold array configured for a continuous tonal range, or result in unacceptable gaps between cells. Consequently, what is needed is a new spot function configured to create dot structures that are optimized for conventional printing processes.